Our invention is adapted to be used in a multiple ratio planetary transmission situated in a vehicle driveline having an internal combustion engine with a throttle control and a hydrokinetic torque converter situated between the engine and input elements of the gearing.
The gearing comprises two simple planetary gear units arranged in a manner similar to the gearing arrangement of the well-known Simpson gear set. Located between the turbine of the torque converter and the input elements of the Simpson gearing is a third simple planetary gear unit with a friction clutch adapted to connect two elements of the third gear unit together for rotation in unison. A friction brake also is used for anchoring a reaction element of the third planetary gear unit. An overrunning coupling establishes one-way torque flow between two elements of the gearing. The brake is arranged in series relationship with respect to the clutch.
A second overrunning coupling in a gear unit of the Simpson gearing is used for the purpose of establishing non-synchronous ratio shifts. Forward engagement is achieved by engaging a forward clutch on a shift from neutral to a drive state. Similarly, a separate reverse engagement clutch is used to establish a torque flow path for reverse. In each instance, turbine speed is used as a feedback signal to initiate the start of the forward or reverse engagement.
The control system of the invention is capable of providing 3-2 downshifts and 2-1 downshifts using a strategy that is vehicle speed dependent when the operator chooses a manual lever position requiring engine braking in each gear. Downshift points always will occur at the same vehicle speed.
A 2-1 shift inhibiting function may provide control ratio steps, for example, from 1.474 to 2.474. The second ratio can be hydraulically locked in second gear when the driver locks the transmission in manual 2 condition. This will permit a so-called "limp home" capability when the transmission electronics fail. Upshifting and downshifting of the overdrive gear unit is permitted when the transmission operates in the manual 2 condition. This allows a 3-2 inhibition function, engaging or disengaging the overdrive gear unit. The two gear ratio step here, for example, may be 0.75.times.1.44 (1.1) to 1.44.
Provision is made for mutually exclusive control of the low ratio brake and the intermediate ratio brake while preventing a planetary gear tie-up because of simultaneous engagement of these brakes. No additional shift valves are required in the control system to accomplish this feature.
Automatic shifts of the overdrive gear unit during manual operation will enhance fuel economy, improve noise level and permit lower engine speeds when engine braking.